Learning outcomes:
understand that organisms are able to respond to changes in their environment
understand that homeostasis is the maintenance of a constant internal environment and that body water content and body temperature are both examples of homeostasis - b - c
understand that a coordinated response requires a stimulus, a receptor and an effector
understand that plants respond to stimuli
describe the geotropic responses of roots and stems
describe positive phototropism of stems
Plants also respond to stimuli. As plants don't have nerves their responses are limited to hormones only. Plants respond to the following stimuli;

- Gravity. Roots grow towards gravitational pull and stems grow away. This is Geotropism.
- Water. Roots grow towards water. This is Hydrotropism.
- Light. Shoots grow towards light. This is Phototropism.

Phototropism is controlled by hormones released by the growing tip of the shoot. Only the tip makes the hormone. If you remove the tip, the shoot stops growing. The hormone made by the tip is called Auxin.

You need to know an experiment that demonstrates Geotropism.

The best example is to grow a runner bean seed in a jam jar. Let the root start to grow downwards then rotate the seed 90’. The root will then start growing at 90’ to its original direction. A more advanced (and less practical) experiment that shows geotropism is that seeds germinating inside spacecraft have roots that grow randomly!

You also need to know an experiment that demonstrates Phototropism. The best example is to repeat the example above, but look at the growth of the shoot!

IAA = Auxin